For ReplicatedBagOStuff to be useful (for sessions), the cache sub-objects need to use the same hashing scheme on both DCs. This is easy for the "master" sub-object but the "slave" sub-object requires either:
a) A local twemproxy instance being pointed to. twemproxy supports redis and tag name hashing
b) A small change to getConnection() in RedisBagOStuff such as allowing a (tag => server) map instead of just a server list in the configuration. Hashing would use the tag and pick the corresponding server.

In any case, the tags would be the eqiad hostnames to avoid re-hashing disruption.